Nitrite and Nitrate Concentrations and Metabolism in Breast Milk, Infant Formula, and Parenteral Nutrition

Abstract

Dietary nitrate and nitrite are sources of gastric NO, which modulates blood flow, mucus production, and microbial flora. However, the intake and importance of these anions in infants is largely unknown. Nitrate and nitrite levels were measured in breast milk of mothers of preterm and term infants, infant formulas, and parenteral nutrition. Nitrite metabolism in breast milk was measured after freeze-thawing, at different temperatures, varying oxygen tensions, and after inhibition of potential nitrite-metabolizing enzymes. Nitrite concentrations averaged 0.07 ± 0.01 μM in milk of mothers of preterm infants, less than that of term infants (0.13 ± 0.02 μM) (P < .01). Nitrate concentrations averaged 13.6 ± 3.7 μM and 12.7 ± 4.9 μM, respectively. Nitrite and nitrate concentrations in infant formulas varied from undetectable to many-fold more than breast milk. Concentrations in parenteral nutrition were equivalent to or lower than those of breast milk. Freeze-thawing decreased nitrite concentration ∼64%, falling with a half-life of 32 minutes at 37°C. The disappearance of nitrite was oxygen-dependent and prevented by ferricyanide and 3 inhibitors of lactoperoxidase. Nitrite concentrations in breast milk decrease with storage and freeze-thawing, a decline likely mediated by lactoperoxidase. Compared to adults, infants ingest relatively little nitrite and nitrate, which may be of importance in the modulation of blood flow and the bacterial flora of the infant GI tract, especially given the protective effects of swallowed nitrite.

Dietary intake and bio-activation of nitrite and nitrate in newborn infants

“…This is true regardless of whether they are receiving breast milk, artificial milk, or parenteral nutrition (8–10). Based on a breast milk intake of 150ml/kg/day and our measurements of nitrate and nitrite concentrations (13 and 0.13 μmol/l, respectively), we have estimated that infants ingest approximately 0.12ml/kg/day of nitrate and 0.0007 ml/kg/day of nitrite from fresh breast milk, which equates to only 5% and 0.6% of the nitrate and nitrite intake of adults (10). A comparison of the average dietary nitrate intake in newborn infants and adults is shown in Figure 2.…”

“…We have also shown that nitrite is oxidized to nitrate in breast milk by an enzyme normally present in milk, lactoperoxidase, leading to even lower levels in milk that has been allowed to sit at room temperature or which has been freeze-thawed (10). Breast milk nitrite concentrations also fall during the first few weeks of life, with the highest levels found in colostrum and decreasing to nearly undetectable amounts in milk collected after the third week postpartum (9,10).…”

“…Breast milk nitrite concentrations also fall during the first few weeks of life, with the highest levels found in colostrum and decreasing to nearly undetectable amounts in milk collected after the third week postpartum (9,10). The levels of nitrate and nitrite in artificial milk vary widely across a range that extends above and below concentrations measured in breast milk, averaging 43 and 0.3 μmol/l, respectively (10) (Figure 2). The recently increased use of nutritional additives for caloric and protein enhancement raises the possibility of an additional source of dietary nitrate and nitrite, although to our knowledge concentrations in these additives have not yet been reported.…”

“…Interestingly, Yazji et al have recently reported that nitrite/nitrate-deficient formula predisposes newborn mice to NEC, and that both the incidence and severity of NEC was ameliorated by nitrite/nitrate supplementation to the formula to achieve levels comparable with that of breast milk (64). However, the clinical relevance of this finding is uncertain given the fact that nitrite and nitrate concentrations of many commercially available formulas are already higher than those found in breast milk (10). Whether manipulation of the decreased levels of ingested and circulating nitrite in the preterm infant would prevent or alter the course of NEC is an area worthy of study.…”

“…( a ) Daily dietary nitrate ingestion, normalized for body weight, is shown for newborns and adults, based on a mean (±SEM) of reported concentrations in breast milk and formula (for newborns) (8–10) and a typical adult diet (4). ( b,c ) Nitrate and nitrite concentrations in total parenteral nutrition (TPN), fresh and freeze-thawed breast milk, freeze-thawed colostrum, and a convenience sample of artificial milk formulas.…”